Donor sheet for pulsed electrical printing

ABSTRACT

A printing ribbon for non-impact electrical printing consists of an insulating base film, having a microcavernous surface formed by microscopic crags having deep rough and irregular pits or valleys between them with fine conductive printing particles loosely retained on and in the surface.

United vStates Patent BEFORE BRU Peshin Sept. 3, 1974 DONOR SHEET FORPULSED ELECTRICAL [5 6] References Cited PRINTING UNITED STATES PATENTS[75] Inventor: John Peshin, Westwood, Mass. 3,550,153 12/1970 Haeberleet a1. 346/74 [73] A gne T e Carters k Company, 3,617,328 11/1971 Newman117/29 Cambndge Mass Primary Examiner-Cameron K. Weiffenbach [22] Filed:Jan. 7, 1972 Attorney, Agent, or Firm-Kenway & Jenney 21 A LN 21 194 1pp 57 ABSTRACT A printing ribbon for non-impact electrical printing [52]525 7 7 consists of an insulating base film, having a microcav- [51] IntCl 844! 1/16 B44d 1/18 ernous surface formed by microscopic crags havingFie'ld 5 74 SB 74 so deep, rough and irregular pits or valleys betweenthem with fine conductive printing particles loosely retained on and inthe surface.

5 Claims, 3 Drawing Figures ISATENTEDSEP 31974 sum 2 or 2 FIG. 3

BACKGROUND THE INVENTION This invention relates to printing ribbonssuitable for use in the non-impact printing (NIP) system described byHaeberle et al in US. Pat. No. 3,550,153 dated Dec. 22, 1970.

The patent of Haeberle et al describes method and apparatus for printingan image upon a recipient sheet by disposing closely adjacent to thatsheet a donor sheet which carries mobile electrically conductiveprinting particles in or on a high resistance medium, and then applyingthrough the thickness of the two sheets a shaped electrical field pulseof short duration corresponding in outline to the image to be printed.In this method the pulse is effective to establish a current flow whichcauses the printing particles to become charged and then transferredfrom the donor sheet to the recipient sheet.

BRIEF DESCRIPTION The object of this invention is to provide a printingribbon, or donor sheet, having a high density of readily removable andtransferable printing material such that images of good tone andcontrast may be produced. These characteristics are provided without theproblems of dusting and smudging, and problems associated with thehandling and utilization of dark colored, finely divided materials, suchas the dry toner employed in the numerous electrostatic printingprocesses.

In brief this invention provides a sheet material which is relativelyclean to handle and not given to smudging or dusting, yet which carriesloosely a relatively large quantity of finely divided printing particlesthat may be readily transferred by the electrical pulses employed in theprinting step.

It is a requisite to the NIP system described by Haeberle et al that theprinting ribbon or donor sheet have a relatively high electricalresistivity, particularly in the lateral directions, so as to preservethe shaped character of the electrical field pulse applied for printing.A conductive sheet would destroy the integrity of the field shape in theregion remote from the shaped electrode, such as between the sheet and aground plate. The printing ribbon of this invention accordingly employsa sheet of relatively insulating material such as paper, polyethyleneterphthalate, polypropylene or the like. To this is applied a base coatwhich develops a rough outer surface of microscopic crags which presentdeep, rough and irregular pits or valleys capable of retaining theprinting particles. This rough surface provides not only a larger areathan would a smooth surface, by a factor of about two to five times(based on squaring the linear measurements of the actual surface outlineand of its projection, made from photomicrographs of cross sections ofthe sheet) but also cavities in the surface in which the particle maylie captive. The printing particles are loosely retained on the surfaceand inv the pits or valleys, essentially by frictional and surfaceforces.

The base coat is conveniently formed form a mixture of an insulatingfiller material of particle size between about 5 and 175 microns and aninsulating resin binder solution of high solvent content, e.g. over 50%by volume on the total mixture, and low resin content, e.g.

5-40% on total solids. The base coat is applied to a thickness suitableto accomodate the largest filler particles in the final coating and isthen dried, whereupon the evaporation of solvent leaves the abovedescribed rough microscopically craggy structure.

To the base coat is then applied the printing particles, preferably byforming a suspension of the particles, e. g. activated charcoal 0.25-15microns in size in a nonsolvent for the base coating, to cover the basecoatingcompletely. After the suspension has dried, the surface is gentlybrushed to remove excess printing particles that lie above the generaltop surface level of the base coating and to distribute the particles inirregular piles scattered over the surface of the base coat. Theprinting particles not only fill the pits of the micro cavernous basecoat but lie all over the surface in loose irregular piles.

A printing sheet as thus formed is well suited for use in the processdescribed by I-Iaeberle et al.

DESCRIPTION OF FIGURES The nature of the printing sheet of thisinvention is shown in the drawing in which:

FIG. 1 is a visualization, based on photomicrographs, of a cross sectionin elevation of the sheet of this invention at a magnification of about1,000x;

FIG. 2 is a visualized enlargement of the circular area of FIG. 1 to amagnification of about 10,000x; and

FIG. 3 is a photomicrograph taken by a scanning electron microscope at1,000x magnification and 45 inclination, of the top surface of the basecoat prior to the incorporation of printing particles.

As shown in FIG. 1 the printing ribbon consists of a thin film 10 havingon one surface a rough coating 12 which provides an outer surface ofmicroscopically deep, rough and irregular contours presentingmicroscopic pits or valleys in which the printing particles 14 areretained. Typically these pits or valleys are about 10-50 microns indepth and across. In FIG. 1 the nature of the coating is illustrated insome detail to show filler particles 15 in the resinous binder 16, theparticles serving to define crags which impart the rough and irregularshaped to the surface. The nature of the outer particulate surface inthe final product is shown in the right hand part of FIG. 1 AfterBrushing, and a visualized enlargement thereof is shown in FIG. 2. Theprinting particles are of the order of 12 microns or smaller and lieloosely on the coating, filling the pits or valleys and also scatteredover the entire surface in irregular piles of more or less looseparticles which remain in place apparently by surface'forces. Thepresence of the scattered piles developed after brushing is believed tobe important in the printing operation in that peaks or pinnacles arepresented where particles would be most readily charged up uponapplication of electrical field pulse, and then dislodged and caused tomove in the field. The field forces tend to concentrate at the peaks orpinnacles and the effect of the field is greatest in these concentratedregions.

The following example sets forth in detail the procedure for preparing aprinting ribbon in accordance with this invention.

EXAMPLE I Estane 57l5 B. F. Goodrich 15 grams EXAMPLE I-Continued Silicasand, -30 microns, Min-U-Sil Pennsylvania Glass Sand Corp.

Methyl Ethyl Ketone (MEK) 25 grams 60 grams opment of the abovedescribed microcavemous surface. The sand particles develop crags withpeaks correspondingto the diameter of the larger particles, and thecrevices between these crags may have a coating thickness only thethickness of residual resin, or of the thickness of smaller particles.In this example the peak thickness of the coating will be up to 50microns. To this is applied a carbon coating prepared by dispersingactivated charcoal (Nuchar CN, West Virginia Pulp and Paper Co.) inmethanol or ethanol in the weight ratio of 15 carbon: 85 solvent. Thedispersion is first ball milled to reduce the particles to 12 microns orless. The carbon dispersion is then coated onto the previously coatedfilm in theamount of 10 ouncesper 3,000 square feet and dried.

The dried coated film is illustrated in FIG. 1 on the left hand sideBefore Brushing.

The final step in the processing is to brush the coating. This may beaccomplished with a rotary brush of 1 inch long bristles ofapproximately 50 denier orless. Brushing is carried out to remove thetop surface of the particles and to cause a distribution in the natureof scattered piles as illustrated at the right hand side of FIG. 1 AfterBrushing and in FIG. 2. Brushing should be carried out to achieve theeffect illustrated but should not be carried out to the point ofexposing the underlying rough surface. An overall coating depth to aminimum of about 10 microns is normal for this example.

The sheet thus formed may be used directly in nonimpact electricalprinting in the manner described by Haeberle et al. Pulses of the orderof 1-2 microseconds and 800 volts will print distinct characters.

The base film and filled resin coating should be of relatively highresistivity such that the configuration of the electrical field pulseapplied for printing is not disrupted or equalized by the interpositionof an electrically conductive layer. Accordingly the'base film can beany of a number of synthetic organic resins as may the'surface coating,and the filler particles should be generally less than 50 microns inmean diameter, and preferably rough and irregular in shape. Theseparticles are generally responsible for the rough and irregular natureof the surface and'should be selected so as to provide the microcavemouseffect .described.

The printing particles should be relatively highly conductive in orderto acquire the necessary electrical charge for printing, should beloosely applied, but not in intimate electrical contact, so that thehigh transverse resistivity of the sheet as a whole is preserved.Suitable materials, in addition to charcoal, that may be employedinclude:

Nigoa Cul CdS PbO co t nu C0 0. CuS PbO C%O CuO U0, Ni ZrN Cu,0

As illustrated and described above the printing sheet of this inventionfeatures a rough and irregular base surface of high lateral resistivityon and in which lie conductive particles of print material. The basesurface serves to retain the print materialwithin its surfaceirregularities, that is to say in the pits and crevices; and, beinghighly resistant laterally, it does not cause equalization of a shapedpulse. The particles on the other hand, readily acquire a chargesufficient to move them from the base surface under the field of theprinting pulse, and in accordance with the shape of that field.

It is accordingly contemplated that the base surface, underlying andretaining the print particles may be any insulating resin with thefiller particles geometrically arranged to present the roughmicroscopically craggy surface described, having an overall high lateralD.C. resistivity, with particles of low resistivity brushed over and inthe crevices. In this connection it is noted that the print particleslisted above include not only highly conductive materials such ascarbon,.but also several semi-conductor materials such as CdS.

A criterion of the surface of the print ribbon or donor sheet of thisinvention is that the print particle carrying surface have alateral D.C.resistivity of at least 10 ohms per square.

Having thus disclosed this invention and described in detail thepreferred embodiment, we desire to secure and claim by Letters Patentthefollowing:

l. A donor sheet useful in pulsed electrical printing, comprising a basesheet having a microcavemous surface with pits-or valleys of about 10-50microns across and 10-50 microns deep, and print particles having aconductivity at least that of a semi-conductor loosely lying on thesurface in said pits or valleys, said particles being about 0.25-l5microns across and present in quantity sufficient to effect printing bytransfer of particles to an adjacent sheet upon imposition of anelectrical pulse of about 800 volts for two microseconds, said sheethaving a lateral surface D.C. resistivity of at least I0 ohms persquare.

2. A donor sheet as defined by claim 1 wherein the microcavemous surfaceof the base sheet presents a total surface at least two times itsprojected area which is based on squaring the linear measurements of theactual surface outline and of its projection.

3. A donor sheet as defined by claim 1 wherein the base sheet surfacecomprises particles of filler 5-175 microns across, embedded in asynthetic organic insulatin resin, said resin being between 5 and 40% byweigit of the total resin plus filler.

4. The method of making a donor sheet suitablefor pulsed electricalprinting comprising the steps of:

mixing coarse particles of filler 5-175 microns across in a solution ofan organic insulating resin having a solvent content over 50% by volumeand a resin tc tl)lntent of 540% by weight based on resin plus coatingsaid mixture onto a base sheet,

drying said coating, thereby forming a microcavemous surface coating onsaid sheet with pits or valleys of about 10-50 microns across and l050microns deep, and

applying loose print particles to the dried coating, said particleshaving a conductivity at least that of a semiconductor, and saidparticles being of size between about 0.25 and 15 microns.

5. The method defined by claim 4 wherein the print particles are brushedon the coating.

2. A donor sheet as defined by claim 1 wherein the microcavernoussurface of the base sheet presents a total surface at least two timesits projected area which is based on squaring the linear measurements ofthe actual surface outline and of its projection.
 3. A donor sheet asdefined by claim 1 wherein the base sheet surface comprises particles offiller 5-175 microns across, embedded in a synthetic organic insulatingresin, said resin being between 5 and 40% by weight of the total resinplus filler.
 4. The method of making a donor sheet suitable for pulsedelectrical printing comprising the steps of: mixing coarse particles offiller 5-175 microns across in a solution of an organic insulating resinhaving a solvent content over 50% by volume and a resin content of 5-40%by weight based on resin plus filler, coating said mixture onto a basesheet, drying said coating, thereby forming a microcavernous surfacecoating on said sheet with pits or valleys of about 10-50 microns acrossand 10-50 microns deep, and applying loose print particles to the driedcoating, said particles having a conductivity at least that of asemiconductor, and said particles being of size between about 0.25 and15 microns.
 5. The method defined by claim 4 wherein the print particlesare brushed on the coating.